1. Technical Field of the Invention
The present invention relates to an electrophotographic image forming apparatus for use in copying machines, laser beam printers and the like, and more particularly to an electrophotographic image forming apparatus capable of obtaining images on both sides of a a transfer material at one cycle of paper feeding process by transferring visualized images on a photoconductor and a transfer belt onto both sides of the transfer material.
2. Brief Description of Related Art
In a conventional copying machine, it has generally been possible to obtain images on both sides of a transfer sheet by firstly transferring and fixing a first image formed on a photoconductor onto one side of the transfer sheet, and then secondly transferring and fixing a second image on a photoconductor onto the other side of the transfer sheet by turning over the sheet.
Meanwhile, U.S. Pat. Nos. 3,697,170 and 3,697,171 disclose an image forming apparatus arranged to transfer images onto both sides of a transfer sheet by using a photoconductor and transfer roller or transfer belt, wherein the function of the transfer roller is basically the same as that of the transfer belt.
For instance, when a transfer belt is used, a first image formed on a photoconductor 31 is firstly transferred onto a transfer belt 32 by a transfer charger 33 as illustrated in FIG. 1. Then, the polarity of toner of the first image transferred onto the transfer belt 32 is reversed by a toner polarity reversing charger 34. Thereafter, a second image which was developed by toner is formed on the photoconductor 31 simultaneously with the first image whose toner polarity has been reversed is returned to the transfer section.
Coordinating with this operation, a transfer sheet 35 is supplied between the photoconductor 31 and the transfer charger 32, and a first image is transferred onto a front side of the sheet and a second image onto a back side simultaneously by the transfer charger 33 to proceed to the next fixing process. The reference numeral 36 in the figure represents a charger provided for tacking the transfer sheet 35 onto the transfer belt 32.
However, in the conventional method of performing transfer and fixing process two times each, it was required to arrange another paper feeding path for reversely transporting the transfer sheet once transferred and fixed thus causing the apparatus to become large in volume by, for instance, 30-40% to result in a considerable rise in manufacturing cost. Another problem is that heat curling can be occurred on a transfer sheet at the time of a first fixing process which causes poor paper feeding at the time of the second transfer and fixing process. Further, considerable time is required for forming images on both, sides since two cycles of paper feeding have to be performed.
In the apparatus disclosed above, the problems aforementioned can be eliminated since it is arranged to simultaneously transfer images on both sides of a transfer sheet. However, the transfer of the first image from the transfer belt to the transfer sheet and the transfer of the second image from the photoconductor to the transfer sheet are simultaneously made by the same transfer charger at the same time under the state that the transfer sheet is placed between the photoconductor and the transfer belt, and therefore, the apparatus is easily affected by the quality and thickness of the transfer sheet at the time of the transfer process thereby causing a spattering of toner and poor transfer. Moreover, when an image formation is required to be made only on one side of a transfer sheet, a proper image can hardly be formed since the transferring efficiency is different from the case when both sides are collectively transferred.